Method and apparatus for the recovery of energy from the spent gases of a catalytic cracking unit



April 19, 1966 E. F. ROELOFSEN E 3,247,129

METHOD AND APPARATUS FOR THE RECOVERY OF ENERGY FROM THE SPENT GASES OF A CATALYTIC CRACKING UNIT Filed July 22, 1963 INVENTORSI EVERT F. ROELOFSEN ARE 6. VAN NES swa THEIR ATTORNEY United States Patent 3 247 129 METHOD AND APPAllATUS FOR THE RECOVERY OF ENERGY FRGM THE SPENT GASES OF A (IATALYTIC CRACKKNG UNIT Evert F. Roelofsen and Arie G. van Nes, The Hague,

Netherlands, assignors to Shell (lil Company, New

York, N.Ii., a corporation ct Delaware Filed July 22, 1963, Ser. No. 295,627 Claims priority, application Netherlands, July 24, 1962, 281,309 6 Claims. (Cl. 252417) The invention relates to a method for the recovery of energy from the spent hot gases from the regenerator of a catalytic cracking unit, from which gases the fluidized catalyst mass to be regenerated has been removed with the aid of one or more separators.

It is known to recover the useful energy from the combustion gases from a fluidized cracking catalyst regeneration system, since these exit gases, from which the catalyst particles have been removed, contain a quantity of combustible components, such as can-hon monoxide, and are, moreover, at a fairly high temperature and at a pressure of several atmospheres. However, these exit gases are not capable of normal, self-sustained combustion when mixed with supplemental air because the concentration of carbon monoxide is too low.

US. Patent 2,307,672 discloses a method of the abovementioned type, in which the combustion gases are further burnt by means of a combustion catalyst and the resultant heated gases are used to drive a gas turbine connected to an air compressor.

According to US. Patent 2,853,455, the gases are further burnt in a combustion chamber, into which fuel gases and supplemental air are also introduced, in order to maintain combustion. The combustion gases then pass through a heat exchanger, where'steam is generated, and are subsequently led through an economizer before being discharged via the stack.

The object of the present invention is to provide a method of the type mentioned in the preamble, which method has certain important advantages over the previously known systems, such as case of control and a favorable energy balance.

This object is attained according to the invention by using a methodwherein the exit gases from a catalyst regenerator are led in to a high-pressure steam boiler (i.e., to a super-charged boiler wherein the gases are at superatmospheric pressure), in which boiler they are burnt under pressure, supplemental fuel and air being added, after which the combustion gases are discharged from the boiler, the outlet temperature of which is controlled, are expanded in at least one gas turbine/ compressor unit, the compressor(s) supplying both the combustion air for the regenerator and the combustion air for the boiler.

Since, according to the invention, the boiler in which the gases are further burnt is arranged ahead of the gas turbine(s) and therefore operates under elevated. pressure on the heat delivery side, this makes it possible to supply high-pressure steam, i.e., steam at approximately 50 atmospheres, with a very satisfactory yield, which steam can be expanded in turbines to a lower but yet moderately high pressure, e.g., approximately 18 atmospheres, and subsequently delivered to the refinerys steam system. The latter pressure is the same as the normal pressure of the steam system in refineries, while, moreover, the partial expansion in the turbines provides sufiicient energy to drive the gas compressors of the cracking unit.

The exit gases from the steam boiler can then drive one or more turbines; the outlet temperature of these gases must, of course, be controlled and it is therefore necessary to provide a suitable temperature-control system. A par- 3,Zd7,l29 Patented Apr. 19, 1966 ticular advantage of the invention is that this control can be realized in a very simple manner.

According to a feature of the invention, the outlet temperature of the exit gases from the boiler is preferably controlled with the aid of a control device which controls the supply of fuel to a combustion apparatus arranged after the heat-transfer sectionin the boiler, as a function of the outlet temperature measured in the gas outlet of the boiler.

If, therefore, the temperature of the exit gases is too low, the combustion apparatus on the outlet side of the boiler comes into operation and insures that the desired temperature level is reached without the heat-transfer in the boiler itself being affected. It is also possible to have the saidcontrol operate in such a way that the combustion apparatus is permanently in operation, but is controllable between given maximum and minimum heat-output levels. The air control for this combustion apparatus may be effected in a known manner, with the aid of one of the known fuel-air control systems, but in principle it may also be eifected directly with the aid of the above-mentioned fuel-supply control device.

If, during operation, the temperature of the exit gases from the boiler becomes and remains too high, even on minimum level or with the combustion apparatus disconnected, the said control device can also, according to the invention, control the supply of cooling water to a waterinjection apparatus provided in the boiler after the tube bundle, this control operating likewise as a function of the outlet temperature of the combustion gases from the boiler.

In the turbine or turbines in which the combustion gases then expand, suificient mechanical energy is generated to drive one or more compressors, supplying both the air for the regenerator and the supplemental air for the boiler.

The quantity of air supplied to the regenerator is preferably controlled with the aid of a flow controller operating a valve in a by-pass line connected to the air line to the boiler. The pressure in the regenerator can be controlled by means of a pressure controller operating a valve in a by-pass line associated with the gas turbines. However, these specific arrangements are optional.

The invention also relates to an apparatus suitable for carrying out the method discussed above. The apparatus according to the invention is characterized by the combination of a centrifugal dust separator connected to the regenerator combustion gas outlet, a steam boiler with a first combustion chamber, provided with an inlet for air and exit gas from the dust separator and an auxiliary combustion device near the said gas inlet, a heat-transfer section, provided with (a) tube bundle(s) for steam generation, and a second combustion chamber, provided with a combustion device, a water-injection apparatus and an outlet for combustion gases, and a number of turbine/ compressor units, with the turbine inlet connected to the gas outlet of the boiler.

The invention will now be described in greater detail with reference to the drawing, the single view of which shows diagrammatically an apparatus according to the in vent-ion.

Referring to the drawing, 1 is the regenerator, represented as a vessel wherein fine spent catalyst particles, coated with carbonaceous matter are maintained as a fluidized bed F. Such catalyst may be introduced continuously at la and discharged at 15. The vessel contains at the top a number of cyclones 2, 3, connected in series and having dip legs immersed in the bed, in which cyclones the catalyst particles being regenerated are separated from the combustion or exit gases which rise from the top of the bed F. These gases are led off via a line 4 and preferably passed through a supplemental cyclone 5, in which these gases are freed from the smallest solid catalyst particles. The ditference between cyclones 2 and 3 on the one hand and 5 on the other is that the particles separated off in the first two cyclones are returned to the process, whereas the particles separated off in the last cyclone are removed since they are not suitable for further use in the unit. As these particles are too fine to be retained in the bed F, they would simply continue circulating through the cyclones without participating in the process, which would represent a useless overloading of the cyclones. The last cyclone 5 has rather the task of purifying the combustion gases sutficiently to ensure that when, in a later stage, these gases are expanded in gas turbines, they do not contain too many harmful solids or components which could impair the turbine blades by erosion or otherwise. It will be understood that in a practical embodiment the cyclone 5 could comprise a battery of small cyclones connected to operate in parallel. The purified combustion gases, which still contain a quantity of combustible components, such as carbon monoxide, are led to a boiler 7 via a line 6. This boiler includes a first combustion chamber 8, a heat-transfer section 9, and a second combustion chamber 10.

The combustion gases are supplied to the first combustion chamber via inlet 11. An air supply line 12 enters the combustion chamber near the same spot to mix supplemental air with the entering gas. This chamber is also provided with an auxiliary combustion apparatus 13, fitted with a line 14 for the supply of auxiliary fuel at a rate determined by a valve 14a, and an air supply line 15 having a valve 15a for controlling the air flow rate. The heat-transfer section 9 of the boiler contains one or more tube bundles, connected on the inlet side to a water supply line 16 and on the outlet side of a steam outlet line 17. The second combustion chamber 10 is provided with a combustion apparatus 18, a water-injection apparatus 19 and a combustion gas outlet 20. The combustion apparatus has a fuel supply line 21, provided with a control valve 22, which is equipped with a valve operator 22a, operated by a controller 23. This controller likewise operates a valve-operator 24a connected to a control valve 24 in the water supply line 25 of the water injection apparatus.

The controller 23 is connected to a temperature-sensing element 23a in the gas outlet line 20 and acts responsively to the temperature of the emerging gas. Depending on the deviation of the measured temperature from the set value for the temperature of the exit gases, which set value can be set into the controller 23, the controller emits control signals so as to maintain the gas temperature as constant as possible. Thus, the fuel valve 22 is opened or closed, or opened further or closed further, in accordance with the known methods of control engineering. This is also the case with the operation of the valve 24, which is opened further or is opened as the gas temperature becomes higher than the desired value, when the valve 22 is in its minimum position or in its closed position.

The air supply line 26 for the combustion apparatus 18 is controlled by a valve 26a having a valve operator 26b, which operates in any known or suitable way. For example, it may be operated as a function of the fuel supply, either by means of a fuel-air control system, or directly by means of the controller 23.

The outlet 20 is connected to a supply line 27 for a first turbine/compressor unit 29, and to a supply line 28 for a second turbine/ compressor unit 31, 32. The turbine 29 is provided with an outlet line 33 and the turbine 31 with an outlet line 34. These outlet lines are connected, together with a by-pass line 35, to a common gas outlet line 36.

An economizer 37 is advantageously fitted in this line 36 to preheat the steam boiler feed water. The heated water is led via a line 38 to the water inlet 16 of the boiler. After passing through the economizer the spent gases are led to the stack via line 39.

The turbine/compressor unit 31, 32 delivers compressed air into a line 40, which line splits into two" branches 41 and 42. The branch 41 is connected to an air distributor 45 Within the re enerator 1 and supplies air to maintain the catalyst fluidized and provide oxygen to burn carbonaceous deposits on the catalyst. The other branch 42 is a by-pass line, fitted with a valve 43. This valve is actuated by a valve operator 43a, which is operated by a flow controller 44 having as its input a signal of the measured rate of air flow to the regenerator, For example, the branch 41 may include a flow measur= ing element 44a, such as an orifice meter. The controller 44 has an adjustable set point and controls the quantity of air supplied to the regenerator at a level determined by said set point by adjusting the distribution of the total quantity of air from line 40 between the branches 41 and 42.

The branch 42 is connected to an air line 46, through which combustion air, supplied mainly by the compressor 30, flows to the boiler. This line 46 is therefore connected to the air supply lines 12, 15 and 26.

If more air is eventually supplied to the line 46 than is required for the process, the surplus is led off, for example via a vent line 47 and control valve 48 to the unit stack. in general, there should be a surplus of air, preferably reasonably small, in order to permit good process control.

The pressure in the regenerator is controlled with the aid of a pressure controller 49. This controller receives a pressure signal from a pressure-sensitive cell 50 in the regenerator and emits a control signal to operate a valve operator 51a. The latter actuates a valve 51 fitted in the by-pass line 35 of the turbines 29 and 31. It will be clear that the further the valve 51 is opened, the more gas will pass through the by-pass line 35 at the expense of the gas stream passing through the lines 27 and 28, and that as a result the turbines will supply less energy. Consequently, the air supply from the compressors will be affected, resulting in a lower pressure in the regenerator. The pressure controller 49 acts responsively to this measured pressure in the regenerator and insures that the position of the valve 51 is adjusted as a function of the difierence between the measured value for the pressure and the desired value set, in accordance with the known methods of control engineering.

The controller 49 may also be connected to operate a valve operator 52a which actuates a valve 52 in a ventline 53 in addition to (or instead of) being connected to the valve operator 51a. The setting of the valve 52 affects the pressure in the air system, and hence the pressure in the regenerator.

As already stated, the plant discussed above is shown only diagrammatically in the drawing. It will be clear that certain designs for or a special arrangement of at least several elements are mentioned only by way of example. Thus, for example, it is not essential for the apparatus for separating gas and catalyst particles to take the form of cyclones. Each gas turbine/compressor unit may of course take the form of a series of units connected in parallel. The boiler may also be duplicated, as may other elements of the apparatus, as is often the case in commercial plants in order to guarantee continuous operation. In addition, the drawing does not give any indications about the relative dimensions of the various parts. The entire instrumentation, including the control apparatus, has been left undiscussed, insofar as it is not of particular interest for the present invention.

We claim as our invention:

1. A method for the recovery of energy from hot exit gases from the regenerator of a catalytic cracking unit which comprises:

(a) maintaining a bed of finely divided cracking catalyst bearing carbonaceous matter as a fluidized bed Within a confined regenerator by passing regeneration air at superatmospheric pressure upwardly through the bed and burning said carbonaceous matter, thereby producing hot exit gas which contains a small amount of carbon monoxide,

(b) separating the catalyst from said exit gas Without substantial reduction in temperature or pressure,

(c) flowing the residual exit gas at substantially said superatmospheric pressure to a high-pressure steam boiler, therein burning the said carbon monoxide by adding supplemental air and auxiliary fuel, transferring heat from the resulting hot gas to heattransfer elements of the boiler, and discharging said gas at substantially said superatmospheric pressure from the boiler,

(d) controlling the temperature of said discharged gas independently of the combustion within said boiler by (l) measuring the temperature of the gas charged to the turbine-compressor set in step (e) hereof,

(2) burning additional fuel and thereby heating said gas after transferring heat to said heattransfer elements, and

(3) controlling the amount of additional fuel burnt in accordance with said measured temperature,

(e) expanding the gas, after said temperature control,

in at least one gas turbine-compressor set and compressing air thereby, and

(f) supplying at least some of the compressed air to said regenerator as regeneration air.

2. A method as defined in claim 1, wherein said step (d) further includes the step of injecting a coolant into said gas after transferring heat to said heat-transfer elements when said measured temperature is too high.

3. Apparatus for the recovery of energy from hot exit gases from the regenerator of a catalytic cracking unit, which comprises:

(a) a regenerator vessel having means for charging and discharging catalyst particles, an inlet for regeneration air, and an outlet for hot exit gas, said vessel being constructed to confine air under super- .atmospheric pressure,

(b) means for separating catalyst from the exit gas without substantial reduction in the temperature or pressure of the gas,

(c) a high-pressure boiler containing boiler tubes and including an inlet and an outlet, arranged for the flow of gas in heat exchange relation with said tubes, including a burner and means for supplying fuel to the burner at a rate in accordance with the said measured temperature, said boiler further comprising (1) burner means including a supply means for auxiliary fuel, and means for admitting supplemental air, both situated ahead of the boiler tubes,

(2) means for measuring the temperature of the gas discharged from the boiler, and

(3) means situated after said boiler tubes for controlling the temperature of the discharged gas in accordance with the measured temperature,

(d) at least one turbine-compressor set connected to expand said discharged gas after control of the temperature thereof, said set having an air intake connected to the compressor thereof, and

(e) a duct connected to the discharge of said compressor for feeding compressed air to the regenerator as regeneration air.

4-. Apparatus as defined in claim 3 wherein said means for controlling the temperature of the gas further includes means for injecting a coolant into said gas and means for controlling the injection of said coolant in accordance with the said measured temperature.

5. A method for the recovery of energy from hot exit gases from the regenerator of a catalytic cracking unit which comprises:

(a) maintaining a bed of finely divided cracking catalyst bearing carbonaceous matter as a fluidized bed within a confined regenerator by passing regeneration air at superatmo'sphe'ric pressure upwardly through the bed and burningsaid carbonaceous matter, thereby producing hot exit gas which contains a small amount of carbon monoxide,

(b) separating the catalyst from said exit gas without substantial reduction in temperature or pressure, (c) flowing the residual exit gas at substantially said superatmospheric pressure to a high-pressure steam boiler, therein burning the said carbon monoxide by adding supplemental air and auxiliary fuel, transferring heat from the resulting hot gas to heat-transfer elements of the boiler, and discharging said gas at substantially said superatmospheric pressure from the boiler,

(d) controlling the temperature of said discharged gas independently of the combustion within said boiler by l) measuring the temperature of the gas charged to the turbine-compressor set in step (e) hereof,

(2) burning additional fuel and thereby heating said gas after transferring heat to said heattransfer elements, and

(3) controlling the amount of additional fuel burnt in accordance 'Wlth said measured temperature,

(e) expanding the gas, after said temperature control,

in at least one gas turbine-compressor set and compressing air thereby,

(f) supplying at least some of the compressed air to said regenerator as regeneration air,

(g) measuring the pressure within the regenerator, and

(h) by-passing around the turbine-compressor set a variable portion of the gas discharged from the boiler in accordance with the measured pressure.

6. Apparatus for the recovery of energy from hot exit gases from the regenerator of a catalytic cracking unit which comprises:

(a) a regenerator vessel having means for charging and discharging catalyst particles, an inlet for regeneration air, and an outlet for hot exit gas, said vessel being constructed to confine air under superatmospheric pressure,

(b) means for separating catalyst from the exit gas without substantial reduction in the temperature or pressure of the gas,

(c) a high-pressure boiler containing boiler tubes and including an inlet and an outlet, arran ed for the flow of gas in heat exchange relation with said tubes, including a burner and means for supplying fuel to the burner at a rate in accordance with the said measured temperature, said boiler further comprising (1) burner means including a supply means for auxiliary fuel, and means for admitting supplemental air, both situated ahead of the boiler tubes,

(2) means for measuring the temperature of the gas discharged from the boiler, and

( 3) means situated after said boiler tubes for controlling the temperature of the discharged gas in accordance with the measured temperature,

(d) at least one turbine-compressor set connected to expand said discharged gas after control of the temperature thereof, said set having an air intake connected to the compressor thereof,

(e) a duct connected to the discharge of said compressor for feeding compressed air to the regenerator as regeneration air,

(f) means for measuring the pressure within the regenerator,

7 (g) means for lay-passing around said turbine-compressor set a part of said gas discharged from the boiler, including flow-control means, and (h) means for operating said flow-control means in accordance with the measured pressure.

References Cited by the Examiner UNITED STATES PATENTS 8 2,853,455 9/1958 Campbell et a1 252417 3,104,227 9/1963 Pfeiffer et al 252417 3,137,133 6/1964 Wilson et a1 6039.02

OTHER REFERENCES Weber: Greater Heat Recovery from Catalytic Flue Gas, The Oil and Gas Journal, vol. 51, No. 23 (1952), pp. 346, 347, 349.

Arden et a1: Disposal of Refinery Waste Gases, The Oil and Gas Journal, vol. 53, N0. 5 (1954), pp. 99101, 109.

BENJAMIN HENKIN, Primary Examiner.

MAURICE A. BRINDISI, Examiner. 

1. A METHOD FOR THE RECOVERY OF ENERGY FROM HOT EXIT GASES FROM THE REGENERATOR OF A CATALYTIC CRACKING UNIT WHICH COMPRISES: (A) MAINTAINING A BED OF FINELY DIVIDED CRACKING CATALYST BEARING CARBONACEOUS MATTER AS A FLUIDIZED BED WITHIN A CONFINED REGENERATOR BY PASSING REGENERATION AIR AT SUPERATMOSPHERIC PRESSURE UPWARDLY THROUGH THE BED AND BURNING SAID CARBONACEOUS MATTER, THEREBY PRODUCING HOT EXIT GAS WHICH CONTAINS A SMALL AMOUNT OF CARBON MONOXIDE, (B) SEPARATING THE CATALYST FROM SAID EXIT GAS WITHOUT SUBSTANTIAL REDUCTION IN TEMPERATURE OR PRESSURE, (C) FLOWING THE RESIDUAL EXIT GAS AT SUBSTANTIALLY SAID SUPERATMOSPHERIC PRESSURE TO A HIGH-PRESSURE STEAM BOILER, THEREIN BURNING THE SAID CARBON MONOXIDE BY ADDING SUPPLEMENTAL AIR AND AUXILIARY FUEL, TRANSFERRING HEAT FROM THE RESULTING HOT GAS TO HEATTRANSFER ELEMENTS OF THE BOILER, AND DISCHARGE SAID GAS AT SUBSTANTIALLY SAID SUPERATMOSPHERIC PRESSURE FROM THE BOILER, (D) CONTROLLING THE TEMPERATURE OF SAID DISCHARGED GAS INDEPENDENTLY OF THE COMBUSTION WITHIN SAID BOILER BY (1) MEASURING THE TEMPERATURE OF THE GAS CHARGED TO THE TURBINE-COMPRESSOR SET IN STEP (E) HEREOF, (2) BURNING ADDITIONAL FUEL AND THEREBY HEATING SAID GAS AFTER TRANSFERRING HEAT TO SAID HEATTRANSFER ELEMENTS, AND (3) CONTROLLING THE AMOUNT OF ADDITIONAL FUEL BURNT IN ACCORDANCE WITH SAID MEASURED TEMPERATURE, (E) EXPANDING THE GAS, AFTER SAID TEMPERATURE CONTROL, IN AT LEAST ONE GAS TURBINE-COMPRESSOR SET AND COMPRESSING AIR THEREBY, AND (F) SUPPLYING AT LEAST SOME OF THE COMPRESSED AIR TO SAID REGENERATOR AS REGENERATION AIR. 